The present invention relates to a fuel cell system and a method for controlling operating pressure of a fuel cell system, and more particularly to a fuel cell system in which water can be preferably reclaimed and accumulated from exhaust gas emitted from a fuel cell and a method for controlling the operating pressure thereof.
According to an investigation by the inventor of the present invention, a configuration mounted in a fuel cell vehicle 101 shown in FIG. 9 is one configuration of fuel cell system.
In the fuel cell system SS of the fuel cell vehicle 101, methanol 17, which is used as a fuel, is steam reformed by a reformer 13 using water 21, thereby generating a reformed gas 23 that includes hydrogen, this being fed to the anode side of the fuel cell 29.
Air 27 from a compressor 25 is sent to the cathode side of the fuel cell 29.
In the fuel cell 29, the hydrogen contained in the reformed gas 23 and the oxygen contained in the air 27 are used to generate electric power.
The hydrogen contained in the reformed gas 23 and the oxygen contained in the air 27 are not completely consumed within the fuel cell 29, a part that is not consumed being sent to a combustor 37, via a condenser 35, as exhaust reformed gas 31 and exhaust air 33.
In the combustor 37, the exhaust reformed gas 31 and the exhaust air 33 are combusted, together with the air 39 from the compressor 25 and the methanol from a methanol tank 15, the heat of the combustion reaction thereof being re-used as a source of heat for vaporizing methanol 17 and water 21 in the reformer 13.
In a system controller 57, based on the pressures of the air and reformed gas detected by pressure sensors 59 and 61 provided upstream from the fuel cell 29, the opening of the pressure adjustment valves 63 and 65 provided downstream of the condenser 35 is adjusted, thereby controlling the operating pressure of the fuel cell SS, this being the operating pressure of the fuel cell 29. When the operating load on the fuel cell system SS is large, control is performed so that this operating pressure is increased so as to achieve the maximum electric power from the fuel cell system SS. When the operating load is small, however, control is performed so that the operating pressure is reduced, so as to increase the efficiency of the fuel cell system.
At the condenser 35, cooling water W is used to cool exhaust reformed gas 31 and exhaust air 33 exhausted from the fuel cell 29, the steam included in the exhaust reformed gas 31 and exhaust air 33 being condensed and reclaimed, the reclaimed water 43 being then returned to a water tank.
In FIG. 9, the bold solid line EL indicates the electric power line, and the EXT indicates the exhaust gas from the combustor 37.
In some cases, air A from the compressor 25 is sent to the reformer 13.
In the above-noted fuel cell system SS, however, the system controller 57, based on the pressures of the air and the reformed gas detected by the pressure sensors 59 and 61. provided upstream of the fuel cell 29, adjusts the openings of the pressure adjustment valves 63 and 65 provided downstream of the condenser 35 so as to control the operating pressure of the fuel cell 29.
For this reason, the reclamation of water by the condenser 35 is dependent upon the operating condition of the system controller 57.
Thus, depending upon the operating condition of the system controller 57, it can be envisioned that there occurs a reduction in the amount of water reclaimed into the water tank 19 from the condenser 35.
Given the above, in order to prevent water in the water tank 19 from running dry, one method that can be envisioned is that of increasing the capacity thereof. However, this is not desirable from the standpoint of layout for installing the fuel cell system SS in a vehicle.
With the fuel cell system SS, because it is necessary to replenish the water tank 19 with water before it runs dry of water, which can happen while the vehicle is being operated, for example, this being undesirable.
Accordingly, the present invention was made in consideration of the above-noted investigation, and has as an object to provide a fuel cell system and a method for controlling operating pressure thereof which substantially do not require replenishment of water used in the fuel cell system and which contributes to the improvement of the practicality thereof.
A fuel cell system according to the present invention comprises: a water tank; a reformer obtaining reformed gas by reforming fuel using water from the water tank; a fuel cell generating electric power using reformed gas from the reformer and gas that includes oxygen; a condenser reclaiming water from exhaust gas from the fuel cell, the exhaust gas reclaimed by the condenser being returned to the water tank; an exhaust gas temperature detector detecting temperature of exhaust gas from the condenser; and an operating pressure controller controlling the operating pressure of the fuel cell system. Here, the operating pressure controller, in response to the temperature of the exhaust gas from the condenser detected by the exhaust gas temperature detector, calculates equilibrium operating pressure of the fuel cell system at which inflow and outflow of the water within the fuel cell system is balanced, in response to operating load on the fuel cell system, calculates maximum efficiency operating pressure at which operating efficiency of the fuel cell system is maximum, and performs control of the operating pressure of the fuel cell system so as to achieve higher pressure of the equilibrium operating pressure and the maximum efficiency operating pressure.
In other words, a fuel cell system according to the present invention comprises: a water tank; a reformer obtaining reformed gas by reforming fuel using water from the water tank; a fuel cell generating electric power using reformed gas from the reformer and gas that includes oxygen; a condenser reclaiming water from exhaust gas from the fuel cell, the exhaust gas reclaimed by the condenser being returned to the water tank; an exhaust gas temperature detecting means for detecting temperature of exhaust gas from the condenser; and an operating pressure control means for controlling the operating pressure of the fuel cell system. Here, the operating pressure control means, in response to the temperature of the exhaust gas from the condenser detected by the exhaust gas temperature detecting means, calculates equilibrium operating pressure of the fuel cell system at which inflow and outflow of the water within the fuel cell system is balanced, in response to operating load on the fuel cell system, calculates maximum efficiency operating pressure at which operating efficiency of the fuel cell system is maximum, and performs control of the operating pressure of the fuel cell system so as to achieve higher pressure of the equilibrium operating pressure and the maximum efficiency operating pressure.
On the other hand, a method for controlling operating pressure of a fuel cell system according to the present invention is applied to a fuel cell system having a water tank, a reformer obtaining reformed gas by reforming a fuel using water from the water tank, a fuel cell generating electric power using reformed gas from the reformer and a gas that contains oxygen, and a condenser reclaming water from exhaust gas from the fuel cell and returning the water to the water tank. Here, the method detects temperature of exhaust gas from the condenser; calculates, in response to the temperature of the exhaust gas from the condenser, equillibrium operating pressure of the fuel cell at which inflow and outflow of the water within the fuel cell system is balanced; calculates operating load on the fuel cell system; calculates maximum efficiency operating pressure at which operating efficiency of the fuel cell system is maximum; and control the operating pressure of the fuel cell system so as to achieve higher pressure of the equillibrium operating pressure and the maximum efficiency operating pressure.